How Do You Calculate Enthalpy Change Using Bond Energies?

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SUMMARY

The discussion focuses on calculating enthalpy change using bond energies, specifically through Hess's law. The user attempts to derive the enthalpy change for a reaction involving 6 C-H bonds, 3 C-C bonds, and other values such as the heat of formation (+53.3 kJ/mol) and heat of atomization (+717 kJ/mol). The calculation involves setting up an equation based on bond energies, leading to an approximate bond energy of 351 kJ/mol for the C-C bond, although the expected answer is 315 kJ/mol. The importance of using conventional notation in calculations is also highlighted.

PREREQUISITES
  • Understanding of Hess's law and its application in thermodynamics
  • Familiarity with bond energies and their units (kJ/mol)
  • Knowledge of heat of formation and heat of atomization concepts
  • Basic algebra skills for solving equations
NEXT STEPS
  • Study Hess's law in detail to understand its application in enthalpy calculations
  • Learn how to accurately use bond energy values in thermodynamic equations
  • Explore conventional notation in chemical equations for clarity and accuracy
  • Practice calculating enthalpy changes using various bond energies and reaction pathways
USEFUL FOR

Chemistry students, educators, and professionals involved in thermodynamics and reaction energy calculations will benefit from this discussion.

Priyadarshini
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Homework Statement


upload_2015-12-27_11-25-13.png


Homework Equations

The Attempt at a Solution


6C-H bonds= 410 x 6 = 2460 kJ/mol
3C-C bonds
H of formation= +53.3 kJ/mol
H of atomisation = +717 kJ/mol
H-H = 436 kJ/mol

as H of formation is 53.3, can't I just do:
2460+3x=53.3?
But then I am not using a lot of the values provided and the answer in not in the options.[/B]
 
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You have to make a "good faith" effort.
 
Bystander said:
You have to make a "good faith" effort.
Actually, I don't know what to do at all. I know that I have make a Hess's cycle so that I can form equations. But I can't figure out the Hess's cycle here.
 
What is Hess's cycle?
 
Bystander said:
What is Hess's cycle?
The total enthalpy change is the same for all the routes the reaction can take to produce the product.
 
Correct. What route are you taking?
 
Bystander said:
Correct. What route are you taking?
Snapshot.jpg

Let the C-C bond energy be x
(3*-717) + (-436*3) + (410*6) + 3x = 53.3
-2151-1308+2460+3x=53.3
-999+3x=53.3
3x=1052.3
x=350.767
approx. 351 kJ/mol

But the answer should be 315kJ/mol.
 
Were you to use conventional notation, you might be more successful.
 
Bystander said:
Were you to use conventional notation, you might be more successful.
What do you mean by conventional notation?
 

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